The weight reduction of a car body has been promoted as a measure to improve the fuel consumption of a car. A stabilizer for suppressing the rolling of a car body during cornering and thus securing the running stability of the car body during high speed running is also one of the subjects of weight reduction. A conventional stabilizer was usually a solid bar manufactured by machining a steel bar into the shape of an end product, but a steel pipe, which is a hollow material such as a seamless steel pipe or an electric resistance welded steel pipe, is often used for the manufacture of a stabilizer for promoting weight reduction.
Improved workability and the soundness of a welded portion are required of a material used for the manufacture of a stabilizer, as the material is formed into a complicated shape or undergoes working such as compression bonding of the ends. In addition, good hardenability must be secured in a heat treatment applied for obtaining high fatigue strength.
The chemical compositions of electric resistance welded steel pipes for hollow stabilizers are described in Japanese Examined Patent Publication Nos. H1-58264 and S61-45688. However, the publications do not describe the regulation of Mo, which is an important element for improving hardenability, and thus the steel pipes based on the publications are unsuitable for securing good hardenability during a heat treatment. In addition, the publications do not specify the quantitative limitations of the contents of N and O, and therefore the control of toughness and oxides in steel is insufficient. Further, none of the publications include descriptions regarding metallographic structure, n-value and hardness, and it is difficult to enhance workability without controlling these items.
A steel pipe of an alloy steel for structural use and a steel pipe of a carbon steel for machine structural use or the like are also used as material pipes for hollow stabilizers in which properties such as workability, the soundness of the welded portion and hardenability are required. However, a steel pipe of an alloy steel for structural use has a problem in the bend formability of the material pipe and a steel pipe of a steel for machine structural use has a problem in hardenability.